Aerosols are the gaseous suspension of fine solid or liquid particles which are also called aerosol particles. In such suspensions, gas and aerosol particles interact with each other in the sense that gaseous substances can condense on the surface of the aerosol particles while simultaneously liquid or solid substances can evaporate from the aerosol particles surface into the gas phase. The equilibrium between the gas and the particle phase is largely driven by the individual compound's saturation vapour pressure.
Aerosol particles usually have a size in a range from 10 nm to 10 μm. Aerosol particles smaller than 10 nm have a large surface to size ratio and therefore grow quickly into larger aerosol particles. Aerosol particles larger than 10 μm on the other hand become too heavy to be suspended in gas for a long time and will eventually fall to the ground. For this reason, the typical size range of ambient aerosol particles is from 50 nm to 2000 nm or 2 μm, respectively.
Methods and an apparatus for analysing the elemental composition of aerosol particles, especially for detecting the elemental compounds of aerosol particles, like metals and black carbon, are known. For example, they are used for analysing anthropogenic (man-made) aerosols and aerosol particles containing trace amounts of metals like for example engineered nanoparticles. They are also used for nanoparticle analysis, since nanoparticles usually consist of a large fraction of metals. Thus, they are employed in atmospheric science, but also nuclear forensics, nanoparticle analysis, environmental analysis like water and air monitoring or quality assurance of food and beverages.
Sampling aerosol particles has traditionally been done using filters or swabs. In this approach, the aerosol particles are collected on filters or swabs and later analysed in an off-line procedure. Over the last 30 years however, several instruments have been developed for analysing the elemental composition of aerosol particles on-line and in real-time. Most of these instruments rely on sampling air directly into an ion source where the aerosol particles are atomised and ionised and then fed from the ion source to a mass analyser. When sampling the air directly into the ion source, most of these ionisations sources first separate the gas phase from the particle phase in several differentially pumped stages whereby the gas phase is diluted by a factor of roughly 1010 by bringing the aerosol particles from atmospheric pressure (approximately 1000 mbar) into a high vacuum or ultra-high-vacuum with a pressure of approximately 10−7 mbar.
Subsequently, the aerosol particles are hit by a laser beam to desorb molecules and atoms from the aerosol particles, and to ionize the molecules or atoms. Upon the laser irradiation, the aerosol particles evaporate and ionize, creating a plasma from the aerosol particle material. If the plasma is hot enough, atomisation occurs and elemental ions can be measured. This class of instruments is usually referred to as aerosol time-of-flight mass spectrometers (ATOFMS).
Multiple versions of such instruments with ion sources which use one or several lasers for vaporising the aerosol particles as well as for ionizing the vaporized substances under high vacuum are for example taught in U.S. Pat. No. 5,681,752 of Kimberley or in U.S. Pat. No. 8,648,294 B2 of Kimberley et al.
These instruments are rather compact and field deployable. However, they have the disadvantage that they require a high vacuum or ultra-high vacuum and are thus extensive and complex equipment. Additionally, they do not allow for measurements with a high precision and reliability because the atomisation and ionisation of the aerosol particles is not very reproducible. One limiting factor of the reproducibility is that the atomisation and ionisation of the aerosol particles depends on the size and the chemical composition of the aerosol particles and on the structure and the surface structure of the aerosol particles. Another limiting factor of the reproducibility is that the type of ions obtained from a specific aerosol particle depends to a large extent on the interaction of the laser beam with the respective aerosol particle. When being ionised, the respective aerosol particle can for example be localised in the fringe region of the laser beam or in the centre region of the laser beam. Depending on this localisation, the obtained ions may range from ions of particle fragments comprising several or numerous atoms to elemental ions comprising only single atoms. One way to reduce these disadvantages is to often re-adjust the laser optics. However, this results in a considerable complication of the equipment's maintenance.
Another way to produce elemental ions from aerosol particles is to use an ion source which uses a gas plasma, e.g. an inductively coupled plasma (ICP) or a microwave induced plasma (MIP) created in a clean plasma gas which is typically argon. In this case, the aerosol particles are desorbed, atomised and ionised in the plasma. Subsequently, the obtained elemental ions are transferred from the ion source to a mass analyser. Since in these ion sources, the plasma is generated independent of the aerosol particles, it is much more reproducible and therefore a more reliable and more reproducible production of elemental ions is enabled.
However, in this approach, the gas phase of the original gaseous suspension of aerosol particles must be exchanged with a clean gas in order to avoid background from gaseous contaminants. This approach is taken in a technique called single particle inductively coupled plasma mass spectrometry (SI-ICP-MS) as taught for example in US 2015/0235833 A1 of Bazargan et al. There, the aerosol particles are transferred from the original gas phase either into a liquid or into a clean gas. The latter is done with a “gas exchange device” as described by J. Anal. At. Spectrom., 2013, 28, 831-842; DOI: 10.1039/C3JA50044F or J-SCIENCE LAB, Kyoto, Japan. Another, even more severe downside of such ion sources and methods for generating elemental ions from aerosol particles is their complexity and need for large amounts of plasma gas supply and large amounts of energy to power the plasma. Consequently, these ion sources and method are not suited for monitoring applications or field applications.
For the reasons mentioned above, the known ion sources and methods for generating elemental ions from aerosol particles have the disadvantage that they either do not enable an efficient and reliable production of elemental ions or require extensive equipment. As a consequence, the known apparatus' and methods for analysing an elemental composition of aerosol particles relying on such ion sources and methods for generating elemental ions from aerosol particles cannot provide reliable and precise results and at the same time be flexibly used for different types of analyses of the elemental composition of aerosol particles, like for example required for on-line and real-time analysis in monitoring applications or field applications.